This application claims the priority of German Application No. 198 17 714.3, filed Apr. 21, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for measuring the positions of structures on a mask surface in which the mask is placed in an image-evaluating coordinate measuring device on a measuring stage that is displaceable in an interferometrically measurable fashion perpendicularly to an optical axis of an image-measuring system. A mask coordinate system associated with the mask is aligned relative to a measuring device coordinate system using alignment marks. The set position of the structures in the mask coordinate system is determined in advance.
A measuring device for performing such a method is described in the manuscript of a paper entitled "Pattern Placement Metrology for Mask Making", by Dr. C. Blasing, Semicon Genf, Education Program, issued on Mar. 31, 1998, the disclosure of which is incorporated by reference herein. One known commercially available measuring device of this type is the Leica LMS IPRO.RTM. system. The measuring device serves in particular for quality control of masks used to manufacture semiconductors. The quality of the mask is becoming increasingly critical in semiconductor chip production. The specifications for the positions of structures (patterns) from one mask to another are becoming increasingly tighter. The measuring device described in the text of the above-mentioned paper can measure the positions of structures relative to specific alignment marks that define the mask coordinate system with a typical accuracy of better than 10 nm. With the aid of these alignment marks, the masks can then be aligned in a stepper for projection on wafer surfaces. Errors made in this process must be accounted for directly in the so-called "error budget" of the lithography process. The mask is aligned in the stepper so that when the respective alignment marks on the mask are illuminated, they are located exactly on top of one another. The steppers however have only one specific area around which the mask can be displaced and rotated for physical alignment.
With ever increasingly strict specifications for all components, the position of the structures on the mask relative to the outer edge of the mask is also becoming an important quality feature of the mask. The positions of the structures relative to the outer edges are termed "centrality" or also "pattern centrality."
The mask is usually located in the lithography device (E-beam or laser lithography for example) using three points (such as banking points or "pins", in order to obtain a reproducible position. Two outer edges of the mask are established with the three points, assuming that these edges are at right angles to one another. These two edges form a reference for the pattern generated by the structures.
Previously, "pattern centrality" was not very significant. This was because the tolerances in the stepper mounts for the masks were so great that the accuracy of the mask lithograph systems met the specifications even without additional measurements. Samples were only randomly measured for pattern centrality for process control purposes. Normal microscopes were used for this purpose, which microscopes were adapted to have the same contact points (typically three) as the lithography systems. The masks were then placed manually by the operator on the microscope stage against the contact points. Special "centrality marks" written on the mask are then measured manually under the microscope relative to the edges of the mask. Provided the distances to the edges remain within a predetermined tolerance range, sufficient alignment of the mask in the stepper with the aid of the alignment marks is guaranteed. The accuracy requirements for measurement are not very high.
With each new chip generation, however, the requirements for accuracy and measuring throughput increase constantly. The accuracy that can be achieved with manual measurement using a conventional microscope is no longer sufficient. Moreover, in manual measurement, too much time is expended on alignment of the mask in the measuring device, finding or locating the structures, and the actual measurement itself. In addition, the mask must be removed from a carrier housing for each measurement in a separate measuring device (the microscope) and, after the measurement, must again be carefully stored in the carrier housing. Each handling process increases the danger of contamination and damage to the mask, such as may occur when placing the mask against the contact points or pins.
Hence, the goal of the invention is to provide a measurement method with which "pattern centrality" can be determined with higher accuracy, increased speed, and reduced risk of damage to the mask.
This goal is achieved according to the present invention by a method of the above-mentioned type wherein in addition to the actual positions of the structures on the mask relative to the mask coordinate system, the positions of at least two external edges of the mask, perpendicular to one another, are measured in the mask coordinate system. Advantageously, the positions of the outside edges of the mask are determined on one coordinate axis from the value of the edge position measured by image evaluation and, on the other coordinate axis, by the current measuring stage position. When two position values are determined for one external edge, and at least one position value for the other external edge, assuming that the external edges are perpendicular to one another, two reference lines can be determined for establishing "centrality."
Advantageously, the positions of the other edges can also be measured, so that it is possible to check the tolerances in the mask plate dimensions, and the alignment of the structures in the mask surface relative to the true center of the mask can be determined.
Since the method uses an image-evaluating measuring system (as opposed to a conventional manual microscope), an image of the external edge of the mask can be stored in the measuring device and an edge position to be measured can be set in an automatic search of the measuring stage. To measure the position of the external edge, imaging optics with a low aperture are advantageously used. If the measuring stage surface is made to reflect, at least in the area of the external edges of the superimposed mask for the imaging beams of the measuring device, then the edge is illuminated in reflected light and sufficient light intensity enters the measuring system to determine the edge of the mask.
By relying on the conventional method for determining "pattern centrality" selected structural elements can be provided on the mask whose positions in the mask coordinate system and relative to the external edges of the mask are determined. However, the positions of all the measured structures relative to the external edges can also be determined according to the invention without special "centrality marks" being provided.
Instead of the position coordinates, the distances to the external edges of the mask can be determined for both the selected structural elements and for the other measured structures.
Advantageously, the present invention is able to measure pattern centrality with greater accuracy in only seconds while eliminating the use of contact or banking points and the need to manually move the mask to a measuring microscope. This further advantageously saves space in the clean room by eliminating the need for the microscope, which is an additional component otherwise necessary in the semiconductor manufacturing process.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.